What should you do first?
Set the thermostat to Heat, raise the setpoint, check the filter, and confirm the furnace switch has not been turned off accidentally. If a CO alarm sounds, gas is suspected, the furnace repeatedly fails ignition, or the breaker trips, leave unsafe conditions and seek the appropriate emergency or repair help.
Safe first checks
Before requesting repair
- Confirm Heat mode and a setpoint above room temperature
- Replace only a safely accessible, correctly sized filter
- Check that supply and return vents are open
- Note any diagnostic light pattern without removing panels
Stop and call
Warning signs that need more caution
- CO alarm, gas odor, soot, or unusual combustion odor
- Repeated ignition attempts without heat
- Burning electrical smell or repeated breaker trip
- Loud boom, grinding, or metal-on-metal noise
- No heat during freezing weather
Possible causes
What may be behind the symptom
Airflow or limit shutdown
Restricted filters, blocked airflow, blower trouble, or overheating can cause a furnace safety limit to stop the heating cycle.
Ignition or fuel-control issue
Igniters, flame sensors, gas valves, pressure switches, venting, and controls can interrupt ignition. These are not DIY repairs.
Thermostat or electrical fault
A thermostat, transformer, control board, switch, wiring, or power problem can prevent a normal call for heat.
Professional diagnosis
What a technician may inspect
- Thermostat demand and control-board fault history
- Airflow, filter, blower, temperature rise, and safety limits
- Ignition sequence, flame proving, gas controls, and venting
- Electrical supply and component condition
- Heat exchanger condition when symptoms or age warrant inspection
Cost context
What can affect repair price?
- The system type, capacity, age, and accessibility
- The failed component and whether compatible parts are available
- Diagnostic or service-call fees in the local market
- After-hours timing and current provider availability
- Refrigerant type, electrical work, and any additional testing required
Common questions
Frequently asked questions
Why is my furnace running but not heating?
The blower may be moving air while ignition, fuel, controls, airflow, or a safety limit prevents normal heat. Check settings and the filter, then arrange diagnosis if heat does not return.
What should I do if I smell gas?
Leave the home immediately. From outside, contact 911 or the local gas authority. Do not operate lights, switches, or phones inside the home.
Is a blinking furnace light useful?
Yes. Note the pattern and model information for the technician, but do not remove panels or bypass safety devices.
Can a furnace be repaired the same day?
It depends on the diagnosis, safe access, parts, and provider schedule. No one should promise completion before the problem is identified.